The present invention relates generally to III-V compound semiconductors and, more specifically to a method of imparting birefringence in a III-V compound semiconductor for sustaining the non linear optical process of second harmonic generation, a birefringent III-V compound semiconductor, and a product of the process.
Lasers have great utility in innumerable applications and their usefulness is well known. Many uses of laser light employed today require, or would benefit from, the availability of a plurality of different frequencies. But, high power output, useful in a wide variety of applications, is limited by wavelength due to the nature of the lasers themselves. For example, CO2 lasers are capable of high power operation but operate at a 10 xcexcm wavelength, which may not be desirable.
Stated differently, it is sometimes desirable for a system to operate at wavelengths that are not produced directly by lasers. Alternatively, the available lasers capable of producing light at the desired wavelength are incapable of producing the requisite power or quality demanded by the contemplated process. In these instances, attempts are made to tune the laser to produce light at different frequencies/wavelengths. This tuning can be achieved in different ways. One such way is to employ the nonlinear optical process of second harmonic generation (SHG) to produce an output light frequency that is double that emanating from the laser. The wavelength is correspondingly halved.
As can be appreciated, the greater the extent to which a laser can be tuned, the wider utility it will have. For example, lasers operating at mid-range infrared wavelengths have great utility in spectroscopy, pollution monitoring, and in military electronic warfare applications, to name a few.
In order to sustain efficient SHG within a material, the material must possess a relatively large coefficient of second order nonlinear susceptibility ("khgr"2). Since efficient SHG also depends upon efficient phase matching within a material, the material should correspondingly possess an appropriate birefringence (xcex94n). Birefringence naturally results in an anisotropic crystalline structure, wherein the index of refraction varies with the orientation of the crystalline lattice with respect to the incident light. Birefringence (xcex94n) is thus quantified as the difference between the refractive indices of light polarized parallel and perpendicular to the optic axis of the crystal. As stated, an appropriate birefringence is desirable in order to sustain efficient SHG which, in turn, will enable efficient tuning of the incident laser light.
The current state-of-the-art material for high-power second harmonic generation of CO2 radiation is AgGaSe2. Recent investigations have been made into the suitability of alloys of AgGaTe2 and AgGaSe as a nonlinear optical material for use in high power tunable solid state laser systems. See, for example, Ohmer et al. Infrared Properties Of AgGaTe2, A Nonlinear Optical Chalcopyrite Semiconductor, Journal of Applied Physics, Vol. 86, No. 1 (Jul. 1, 1999) pp. 94-99. An advantage of these materials is that they intrinsically have a high birefringence due to their anisotropic crystalline structure. A disadvantage lies in the average "khgr"2 value. Another disadvantage lies in the fact that the orientation of the crystalline lattice with respect to the incident laser light is critical and must be precisely controlled. Yet another, perhaps even greater disadvantage to the use of is that the cost of AgGaSe2 and AgGaTe2 is high, and availability of these materials is limited, rendering a their widespread use in laser doubling systems problematic.
A need exists therefore for an improved nonlinear optical material for use in tunable solid state laser systems such as laser doubling. Such a material would be relatively inexpensive, angle insensitive and enable efficient SHG.
Accordingly, it is a primary object of the present invention to provide a method of imparting birefringence in a III-V compound semiconductor for sustaining the non linear optical process of second harmonic generation and product of the process thereby.
Another object of the present invention is to provide a birefringent III-V compound semiconductor capable of achieving non-critical phase matching for sustaining the non linear optical process of second harmonic generation.
Yet another object of the present invention is to provide a method of imparting birefringence in a III-V compound semiconductor capable of achieving non-critical phase matching.
It is still another object of the present invention to provide a birefringent III-V compound semiconductor requiring no angle tuning with respect to incident laser radiation.
These and other objects of the invention will become apparent as the description of the representative embodiments proceeds.
In accordance with the foregoing principles and objects of the invention, a method of imparting birefringence in a III-V compound semiconductor for sustaining the non linear optical process of second harmonic generation, a birefringent III-V compound semiconductor, and a product of the process are described.
III-V compound semiconductor materials have a very high "khgr"2 values, making them potentially very useful for sustaining nonlinear processes. Additionally, many III-V compound semiconductor materials have excellent transmission characteristics in the infrared region, from 1 to 15 xcexcm. For example, InP is a material with excellent transmission properties in the entire mid-infrared region from 1 to 13 xcexcm and it has a high "khgr"2 value of 287 pm/V. And, III-V compound semiconductors are plentiful and relatively inexpensive and thus would appear to be ideal candidate materials for SHG in the infrared region, such as for CO2 laser frequency doubling applications.
The problem with using III-V compound semiconductors in this application is that by nature they are not birefringent. As a result they are not capable of achieving phase matching conditions which are a requisite to efficient nonlinear optical processes such as SHG. Phase matching is essential for effective SHG because it avoids the possibility for destructive interference of the pump and generated waves.
According to an important aspect of the present invention, a III-V compound semiconductor can be made birefringent for sustaining the nonlinear process of SHG by the introduction of a predetermined number of micropipes into the material. The micropipes create an artificial birefringence within the III-V compound semiconductor enabling it to achieve phase matching conditions. Advantageously, due to the large difference between the index of refraction of the air in the micropipe and the index of refraction of the semiconductor material, a relatively small number of micropipes are needed to create a significant birefringence.
According to the method of the present invention, the number of micropipes necessary to achieve the desired birefringence in the material can be calculated by first selecting a generated wavelength value to be output by the second harmonic generation process and determining a corresponding pump wavelength value. Next, using the Sellmeier equations, a bulk index of refraction value of the III-V compound semiconductor for the generated wavelength value is calculated as is the bulk index of refraction for the pump wavelength value. Next, a range of fill factor density values for the III-V compound semiconductor is chosen. The perpendicular index of refraction value and the parallel index of refraction value are calculated for each of the fill factor density values. Then for each fill factor density value, the perpendicular index of refraction value is subtracted from the parallel index of refraction value to obtain a difference value. The optimum fill factor value is then chosen by selecting the fill factor value corresponding the lowest difference value. The fill factor value is then translated into a number of micropipes to be fabricated within the material. The micropipes can then be fabricated in the material by several methods, such as anodic etching, focused ion beam methods and dry etching methods such as reactive ion beam etching.
Advantageously, and according to an important aspect of the present invention, the III-V compound semiconductors modified by the method of the present invention exhibit the highly desirable non-critical phase matching condition during operation, dramatically enhancing the efficiency of operation by eliminating walkoff and the requirement for angle tuning of the material with respect to the incident beam in order to achieve phase matching.